Vacuum tube amplifying circuits



March 29, 1932. R. s. HAYES 1,851,905

VACUUM TUBE AMPLIFYING CIRCUITS OOO CYCLES March 29, 1932.

R. S. HAYES VACUUM TUBE AMPLIFYING CIRCUITS Filed Nov. 50 1927 2Sheets-Sheet 2 |NvENTo R Ralph zlcgyea,

v ATTORNEY Patented Mar. 29, 1932 RALPH S. HAYES, OF ARDMORE,PENNSYLVANIA VACUUM TUBE AMPLIFYING CIRcUrs Application yfiled November30,` 1927. Serial No'. 236,804;

'and more particularly to a new and improved method of obtaining adesired non-linear .Li gain-frequency characteristic in vacuum tubeamplifiers employed in certain radio receiving and phonograph pick-upcircuits.

In the present and known types of radio receiving sets, it is a wellrecognized fact that the very high tones and the very low tones frombroadcasting stations areY reproduced poorly, and this has been found tobe due to theusual flat audio amplifier characteristics of such setsdisclosing a marked deficiency from perfect tone production orreproduction, which is causedby certain inherent forms of frequencydistortions, wherein'the higher tones are lost by reasonof radiofrequency tuning, which is called side-band cutting, and of therelatively poor efficiencies of loud speakers at the higher tones; andthe lower tones by reason of the intentional softening downof the bassinstruments at the broadcasting stations orkstudios, of the icomparatively attenuated transmission of such tones through theamplifiers of many stations, and of the relatively poor efficiencies ofloud speakers or the pick-up attachments for phonographs for-the basstones. Although these facts have been known in the practice, no attempthas heretofore beenr made commercially kto correct or. cure both thesedeficencies, principally on account of the complex apparatus thought tobe necessary or required for the purpose, not only in the audio but alsoin the radio frequency part of the system.

It is, therefore, the principal object of the invention to provide for asimple and comparatively inexpensive method and means for effectivelyovercoming the serious disadvantages occasioned by the presentintentional softening down of the low tones -of the bass instruments inbroadcasting stations or studios as aforesaid, and the unnecessarydeficiencies in receiving sets to properly and perfectlyreproduce boththe high and low` tones from such stations or studios.

Another object ofthe invention is to provide for a method and means ashereinbefore generally characterized, wherein the usual fiat audiofrequency characteristics of radio receiving set-s will be modified intodished gain-frequency characteristics, such as will greatly improve thequality of tone reproduction and otherwise correct or compensate for thedeficiencies aforementioned throughout the range of the higher and thelower frequencies, providing, in other words, a tone control circuit.

A. further object of the invention is Vto provide for an equalizingstage of audio frequency amplification to be used in connection withradio receiving circuits as and for the purposes aforesaid and also inconnection with electrical pick-ups for phonographs.

In carying out the present invention, the equalizing stage of audioamplification under contemplation is constituted in a stage of doublytuned audio frequency amplification,

having two frequencies of maximum gain, which are obtained by theparallel resonance of two tuned parallel or tandem circuits, and onefrequencyof minimum gain obtained by the resonance of the smaller'inductance with the larger capacity. This double parallel resonancecircuit has the unique and appropriate qualities to accomplish thedesired results, in that it gives two peaks in the amplifica-tioncurve,.dependent upon the parallel resonant frequencies selectedtherefor and a minimum point between the two peaks dependent onr theseries resonant frequency. The invention includes a higher tuned circuitand a lower tuned circuit, and at one particular frequency there is alseries resonant effect of minimum gain created by the inductance of thehigher tuned circuit, and the `capacity of the lower tuned circuit. Inthe practicalapplication of the invention, a wide range of selectiveconstants are avail'- able to give the proper parallel resonant andseries resonantfrequencies.

lith the foregoing and other equally important objects, purposes andadvantages in View, the invention resides in the certain new and useful`combinations and arrangements `of instrumentalities and circuits as willbe hereinafter more fully described, set forth 00 in the appendedclaims, and illustrated in the accompanying drawings in which:

Figure 1 is a plot of the gain frequency characteristic of theequalizing amplifier stage in accordance with the exempliied Vap,-plication of the invention,

Figure 2 is a preferred circuit arrangement, and Y i Figures 3, 4, 5 and6 are slightly modified but equivalent net worlrs or circuitarrangements.

Referring to the drawings, wherein simila'r characters of reference`designate cor- Y responding elements or parts throughout the ,severalviews thereof, and more particularly to Figure 2, 10`and 11 indicate twothreeelectrode vacuum tubes constituting preterably the first and secondstages of audio frequency ampliiica'tion of a conventional form ot radioreceiving circuit. ln this particular circuit the grid 12, of the tube10, is connect ed Vby a conductor 13, and the filament 14 thereof byaconductor 15 for the application thereto cia band of audio frequenciesfrom a suitable source (not shown). The plate 16, of the tube 10, is, inturn, connected by a conductor 17 through an interposed condenser 18 tothe grid 19 of the tube 11. Connected to the con ductor 17 at a pointbetween the condenser 18 and the 'grid 19 is one end of a resista-nce20, from the 'opposite end of which a conductor 21 leads to and connects-with the ilainent. 22 of the tube 11. The resultant amplifiedfrequencies from the tubes 10 and 11 are available from the plate 28, ofthe tube 11, through the conductor 24 leading from the plate 23.

To create the two peaks in the impedance or audio amplification curve,as aforesaid, the invention contemplates the `provision of two parallelresonant circuits, designated f1 and f2, coupled in tandem lor `inVseries parallel, as shown in Figure 2. Une side of the circuit f1 iscoupled by a conductor 25 to the conductor 17, between the Vplate 16,'ofthetube 10 and the condenser 18, -and Vthis circuitjinclu'des an.inductance L1, a resistance RL and a condenser C1, all connectedinseries in a local circuit.Y The conductor 25 connects the circuit `f1 at1a point betweenithe condenser C1 and the inductance L1. The'oppositeside oi the circuit f1, between the resistance Rl and the condenser C1is connected by a conductor 26 to one side of the second circuit f2,

5 which consists of the inductance L2, `the re- `from a point betweenthe resistance R2 and Athe condenser C2. R1 yand R2 representfthenecessary coilresistance in the coupledcin cuits f1 and f2. Theconductor 27 is lead to land is connected with one terminalfof a'resistance R3, which has its other terminal Veach end Vof this band 25or more decibels to a maximum at -about Ll() cycles, and at 5000 cycles,two .peaksare desired or necessary in the impedance Yfrequency curve,and these a-re produced by 'the two circuits f1 and ,t2 coupled intandem or series parallel substantially as shown. Further, the 5000cycle tuned circuit f1 should have a ratio of L VU not too large nor toosmall, since the sharpness of resonance 'varies at this ratio, also, theminimum impedance of the two circuits f1 and fzshould be between 200 and7 00 cycles, since the frequency of this minimum inipedancelis'de'termined by the resonant point of the impedance L1 ofV theOOOcycle parallel circuit f1 with the capacityCg of the L10 cycle circuit,which capacity G2 should preterably he between .2 mi". and .4l mi?.AHaving determinedfthe valueof C2, the value ot L2 /canrbedetermin'edfrointhe fact that L2 and C2' `must Vbe resonant at approximately 40cycles. Finally,this'resistance R3, in the circuit f", which isconnected in series with the two reson'antcircuits f1 and f2, serves to'fiatten `out fthe resonant minimum gain ot the stage. The resistance inthe inductances R1 fand fRz of the 4two parallel-'circuits f1 and f2, aswouldbe usedvfor such an equalizing'stage, "as hereincontemplated, havebeen found to have a negligible ieffect, and taking into 'account'rather large resistancesas compared to computations neglecting suchresistances, the

following tendencies have been noted (a) that there is a. slightreduction in the maximum gain, and (b) a comparatively slight increaseinfminimuin gain.

As aneXample of the 'practical application of the invention, afterhavingdetermined the gain of an 'amplifier'stage to be approximately:

p, Z Voltage amplification ZH! rp ywhere u: tube amplificationvconstant; Z:

impedance of the output circuit;

and r.=plate resistance oi the tube. it was evident that the gain of astage could be de- 'signedY to` vary with :frequency :by suitablychoosing the output impedance Z, since u and rplareconstants ofthe tube,and inria high mu ftype'fofftube, asl an linstanceytliese con stants areapproximately 20 and 40000, respectively. Now, by trial computations, itwas found that L1 should be from .25 to .50 henry for proper results,but, for present purposes, L1, in the circuit f1 (Figure 2) is indicatedat a value of .25 henry, and Cl at .00404 mf., tuning to 5000cyc1es,'wl1ile in the circuit f2, L2 has been taken at a value of 82henries, for convenience, and C2 in three steps having values of .2; .3and .4 mf. The

resistance R3, in series with the two circuits f1 and f2, and serving toHatten out the resonant minimum gain of the stage, has been indicated ata value of 2100 ohms in order to show a minimum gain of zero.

The values of the several condensers C, impedances Z and the resistanceR; the frequencies, both resonant and anti-resonant, obtaining in thecircuits fland f2,

in circuits f2 are to be computed as follows, by way of example 1.Coupling resistance (Ra) for zero gain R3 Ohms 2. Resonant .frequenciesfor 40 cycle circuit (a) Assuming 02:,.2

' (b) Assuming 02:.3

fummo# 62808203 (c) Assuming 02:.4

3. Resonant frequencies of L1 with C2 38 cycles 32 cycles 28 cycles (a)Assuming 02:.2

fuman# v(S28/.250.2

(b) Assuming U2=.3

=710 cycles (c) Assuming 02:.4

1000 y f f3 500 cycles 4. The impedance Z1 of a'parallel resonant and ofthel gains with the condensers of different rating` 35' circuit iscomputed .by the usual formula for the impedance of anr inductanc'e inparallel with a capacity (neglecting coil resistance), 1. er vSuchcalculations result in the following impedance values for thevarious tuned circuits at various frequencies, where- Z1=impedance of5000 cycle tuned circuit, ZM: impedance of 40 cycle tuned circuit with5. The voltage amplification and the equivalent gain in decibels arecomputed from the relations Voltage arnplification=gZ y f Decibels 2Olog Zrp.

Using the previously mentioned value of R3, 2100V ohms, and mu equals20, there results the following decibels gain for` the entire amplifyingstage for the three values of()2 In the use yof this equalizing stage,the following agreeable impressions obtain, (a) an effectivcsharpeningof the radio frequency tuning at'tlie cut-off frequencies, which gives Ythe effect, of band-pass radio frequency tun-v .-i.

ing in a simpler manner; (b) the bass fundamentals are reproduced withan appreciable increase of the naturalness in reception; (c) a naturalsparkle and brilliancy tothe program due to the improved reproduction ofthe higher harmonics; (d) in speech reception, thesounds Z, S, F, TH andV are reproduced with clearness and better understanding; (e) a feelingis experienced of volume without loudness,-and (f) an apparent inccrease in the broadcasting station or studio reverberation, which `givessomewhat of a binaural eHect to the reproduction. ,c

It is to be noted that the equalizing stage, as designed, assumes a fiatcharacteristic from the broadcasting station, which, however, is notalways the case, but, if the low tones are only partially transmitted,the using ot' the lower Values oi C2 is of considerable benelit. Again,if there is an vover emphasis of the higher tones from the broadcastingstation, as sometimes may be the case, then the 5000 cycle circuit f1 ofthe equalizingstage should be'partially shunted by a resistance.

Aln, fact, bya proper selection of the constants of the circuits, bypartially shunting the circuits with resistances to dampen their effect,any desired Variation of the dished gain frequency characteristicv maybe obtained.

lt is to be further noted that full beneiit from this equalizer stagecannot be obtained without sutlicient power handling capacity in thefinal power amplifier and in the loud speaker. y

In Figures 4, and 6, are shown net works or circuits coupled in seriesparallel or in tandem, each olf which net works have` been found to besubstantially equivalent to the two parallel resonant circuits f1 andf2, as illustrated .in Figures 2 and 3.

In Figure 4,'the lead 256 connects a conductor extending between oneterminal of an impedance coil L5, of a by-pass condenser C5, in thecircuit f6, and an impedance coil L6, in the circuit f7, with its pointof connection lbeing preferably between the terminal of the condenser C5and that of the impedance coil The other terminals of the impedance coil'L5 and the condenser C5 being connected one to the other by aconductor, which, in turn, is, at an intermediate point, connected toone terminal of ai condenser C6, inthe circuit f7 denser C6 being, inturn, connected by a conductor tothe other` terminal of the impedance iconductor vforming one side ot the circuit f, the latter conductor beingextended between and connected to'one terminal of a condenser` CS andtot an impedance coil LS. The other terminal of the impedance coil L7 isconnected to one terminal of a condenser Cain the circuit 74S, while theremaining` terminal of the latter condenser@7 is, in turn,

i connected tofan intermediate pointon a conduction extendingbetween theremaining ter- Tlie remaining` terminal of the latter con-V pedance coilL10, of the circuit 7m. The other terminal of the condenser C9, orp lthecircuit f1", is connected by a conductor to one ter# minal ot animpedance coil L9, which has its other terminal connected to anintermediate point on a conductor extending between the remainingterminals of the condenser C10 and `the impedance coil Lm, of thecircuit fn.

The conductor 27d leads from plus B battery, and connects anr inte`mediate point on the conductor extending vbetween the condenser C9 andthe impedance coil LD of the circuit flo.

In employing any one of these modified equalizing circuits with areceiving circuit hook-up, as described in conjunction with the first orpreferred term of equalizing stage or circuit, the conductors 25a, 25?),250 or 25d will be connected to the bus-wire or conductor 17 after themanner of the conductor 25, and the conductors 27a, 27?), V27e and 27dto the plus B battery connections of the said liookup aforesaid, afterthe manner ci the conductor or lead 27.

It is to be noted of these modified circuits that, after determining theproper values of the several capacities, impedances and resistancescomprising the same, in accordance with the usual computations ot suchValues and in the manner et the examples given in connection with theiirstinstance of the invention, the same are capable of beingsubstituted, with equal eliiciency in use, one for the other or for thefirst of the similar circuits described herein. g I

Vlith'out Yfurther description, it is thought that the features andadvantages of the invention will be readily apparent to those skilled inthe art, and it will oi course be understood that changes in the form,proportion and minor details ot construction may be iesorted'to, withoutdeparting from the spirit of the invention or its scope as claimed.

-Iaving thus fully described'the invention,

what is claimed is:

l. In' a radio receiving system, means for equalizing the audioainplilication of the system, comprising an auxiliary stage of audiofrequency amplification formed ot circuits electrically connectedtogether and coupled in on the output'side of the usual audio frequencyamplifying means of the said system, said circuits being tuned in suchVamanner vthat peaks spaced relatively tarV apart and isa correspondingin number to that of said circuits are produced in the impedance curveof the system, which act to modify the-usual flat audio frequencycharacteristic of the system into a dished gain-frequencycharacteristie.

2. In a radio receiving system, means for equalizing the audioamplification of the system, comprising an auxiliary stage of audiofrequency amplification formed of two resonant circuits electricallyconnected together and coupled in on the output side of the usual audiofrequency amplifying means of the said system, said circuits being tunedin such a manner that two peaks spaced relatively far apart are producedin the impedance curve of the system, which act to modify the usual fiataudio frequency characteristic of the system into a dishedgain-frequency characteristic.

3. In a radio receiving system, means for equalizing the audioamplification of the system, comprising an auxiliary stage of audiofrequency amplification formed of two resonant circuits electricallylconnected together in series-parallel and coupled in on the output sideof the usual audio frequency amplifying means of the system, saidcircuits being tuned in such a manner that two peaks spaced relativelyfar apart are produced in the impedance curve of the system, which actto modify the usual fiat audio frequency characteristic of the systeminto a dished gain-frequency characteristic.

4. In a radio receiving system, means for equalizing the audio frequencyamplification of the system, comprising an auxiliary stage of audiofrequency amplification formed of a plurality of resonant circuitselectrically connected together in series-parallel and coupled in on theoutput side of the usual audio frequency amplifying means of the saidsystern. said circuits being tuned in such a manner that the interactionthereof will produce peaks at different ends of the resonance Vband inthe impedance curve of the system, which modify the usual flat audiofrequency characteristic of the system into a dished gainfrequencycharacteristic.

5. In a radio receiving system, means for equalizin the audio frequencyamplification of the system` comprising an auxiliary stage of audiofrequency amplification formed of a resonant circuit tuned to a highfrequency and a second resonant circuit tuned to a comparatively lowfrequency, said circuits being electrically connected together inseriesparallel and coupled in on the output side of the usual audiofrequency amplifying circuit of the said system, whereby two peaks areproduced in the impedance curve of the system, which act to modify theusual flat audio frequency characteristic of the system into a dishedgain-frequency characteristic.

6. In a radio receiving system, means for equalizing the audio frequencyamplification of the system, comprising an auxiliary stage of audiofrequency amplification formed of resonant circuits having differentimpedance andcapacity values, said circuits being electrically connectedtogether in series-parallel and coupled in on the output side of theusual audio frequency amplifying circuit of the said system, and tunedin such a manner that two peaks spaced relatively far apart are producedin the impedance curve of the system which act to modify the usual flataudio frequency characteristic of the system into a. dishedgain-frequency characteristic.

Signed at Philadelphia in the county vof Philadelphia and State ofPennsylvania this 21st day of November, A. D. 1927..

RALPH S.y HAYES.

